Schottky-barrier device with locally planarized surface and related semiconductor product

ABSTRACT

The present disclosure is related to alleviation of at least some of the drawbacks of the previously known implementations and to provide an improved alternative. Generally, at least some of the embodiments are related to a high voltage power conversion semiconductor device, in particular a SiC Schottky-barrier power rectifier device, having a planarized surface.

RELATED APPLICATION

This application claims priority to and the benefit of application No.61/665,090, entitled, “Schottky-Barrier Device with Locally PlanarizedSurface and Related Semiconductor Product”, filed on Jun. 27, 2012,which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the field of high power semiconductordevice technology and, in particular, to high power silicon carbide(SiC) based devices such as a SiC Schottky-barrier power rectifierdevice and a method of manufacturing such power rectifier devices.

BACKGROUND

Silicon carbide Schottky-barrier devices are high-performance powerdevices having lower power losses than conventional silicon devices andcan operate at higher switching frequencies. SiC presents the advantagesof having a high breakdown electric field, high thermal conductivity andhigh saturated drift velocity of electrons. SiC is a wide bandgapsemiconductor and may advantageously be used for manufacturing devicesfor low-loss power conversion applications, such as rectifiers.

Generally, power rectifier devices may be manufactured from epitaxiallygrown SiC layers. Epitaxial SiC layers usually present a number ofirregularities due to dislocation defects, such as growth pits,hillocks, and growth steps. Such morphology defects may result inregions of electric field concentration increasing the probability ofelectron tunneling from the Schottky metal into the SiC drift layer,thereby increasing leakage currents at high blocking voltages.High-temperature stages of the manufacturing process of the powerrectifier device, such as for example implant anneal, might also resultin surface roughening due to diffusion of silicon and carbon along thewafer surface.

The pattern of the electric field concentration depends on theconfiguration of the irregularities at the SiC surface. A needle-shapedpit, having a relatively narrow width as compared to its depth along thedirection of epitaxial growth, may for example cause a high localconcentration of the electric field. A shallow pit, having a relativelylarge lateral extension, may on the other hand result in a smallerextent of electric field concentration. Curvature of radius and depth ofthe pit, the applied voltage, and the thickness of the doped SiC layerare examples of parameters that may affect the leakage currents of thepower rectifier device.

Thus, it would be desirable to provide a power rectifier device, and acorresponding method of manufacturing, wherein a surface of the driftlayer has an improved smoothness.

SUMMARY

The present disclosure is related to alleviation of at least some of theabove drawbacks of the prior art and to provide an improved alternativeto the prior art.

Generally, at least some of the embodiments are related to a highvoltage power conversion semiconductor device, in particular a SiCSchottky-barrier power rectifier device, having a surface (of the driftlayer) with improved smoothness. Further, at least some of theembodiments are related to a method of manufacturing a power rectifierdevice with reduced leakage currents.

At least some embodiments include a power rectifier device and a methodhaving the features described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages, will be better understood through thefollowing illustrative and non-limiting detailed description ofpreferred embodiments, with reference to the appended drawings, inwhich:

FIG. 1 schematically shows a cross section of a power rectifier deviceaccording to an embodiment;

FIG. 2 schematically shows a top view of a rectifier device according toan embodiment;

FIG. 3 schematically shows a top view of a power rectifier deviceaccording to another embodiment;

FIG. 4 is a schematic cross section of a power rectifier deviceaccording to an embodiment;

FIGS. 5 a-5 d schematically show a planarization process of a powerrectifier device according to an embodiment;

FIGS. 6 a-6 b schematically illustrate a pit of a surface of the driftlayer before and after a planarization etch; and

FIG. 7 is a block diagram illustrating a method of manufacturing a powerrectifier device according to an embodiment.

All the figures are schematic, not necessarily to scale, and generallyshow parts which are necessary in order to elucidate embodiments,wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION

According to at least a first general aspect, a power rectifier deviceis provided. The power rectifier device includes a drift layer includingsilicon carbide and a Schottky electrode disposed on the drift layer.The drift layer and the Schottky electrode provide a Schottky contact,wherein the drift layer has a planarized surface such that the depth ofany pit of the surface of the drift layer is less than approximatelyD_(max)=E_(b)/F_(a), where E_(b) is the metal-semiconductor energybarrier height and F_(a) is the avalanche breakdown field.

According to a second general aspect, a method of manufacturing a powerrectifier device is provided. The method can include the forming a driftlayer including SiC, forming a sacrificial layer on a surface of thedrift layer, transferring the morphology (or structure) obtained in thesacrificial layer to the surface of the drift layer, and forming aSchottky electrode on the drift layer, wherein the Schottky electrodeand the surface of the drift layer provides a Schottky contact.

Some embodiments makes use of an understanding that by removing pitshaving a certain depth, a negligible (or at least reduced) effect ofpitting on breakdown performance of the power rectifier device may beobtained. A suitable maximum depth of the pits may be defined as theratio between the metal-semiconductor barrier energy height and theavalanche breakdown field.

Such a surface may be obtained according to a method of manufacturing inwhich the surface morphology of a sacrificial layer is transferred tothe (pitted) surface of the drift layer. Advantageously, the sacrificiallayer has a smoother surface than the original (pitted) surface of thedrift layer.

The power rectifier device may be a SiC Schottky barrier power rectifierdevice, such as a diode, or a semiconductor device including at leastone Schottky-barrier junction.

With the term “pit” should be understood as any hollow, hole orindentation in the SiC surface. A pit may be related to a morphologicaldefect, e.g. a crystallographic dislocation such as a screw dislocationand an edge dislocation occurring during epitaxial growth of thesubstrate, or a post-growth defect caused by processing (subsequent toepitaxial growth) such as diffusion of carbon and silicon atoms duringanneal, or ion bombardment induced damage.

A pit may include an aperture located at the surface of the drift layerand extend via side walls to an opposite bottom. The pit may extend inthe drift layer in the direction of epitaxial growth and the length ofthe extension may be referred to as the depth of the pit. The lateralextension of the aperture may be referred to as the width of theaperture and may be circular or of any other shape. The bottom of thepit may for example be flat, or form a sharp angle defined by thetapering of the sidewalls. The bottom of the pit may also be defined bya width, or a radius of curvature. The depth of the pit, the width ofthe aperture and the bottom, and the tapering of sidewalls, define theshape of the pit.

As the Schottky electrode is disposed on the drift layer, the metal maypartly or fully fill up a pit and thereby create a metal projection inthe semiconductor material. The shape of the portion jutting out fromthe metal layer may correspond to the shape of the pit and defines theelectric field concentration.

Reverse currents of power rectifier devices are dominated by tunnelingwhich is affected by barrier height and the surface morphology at themetal-semiconductor interface. Pits at the surface, extending into thedrift layer, may cause electric field concentrations in the drift layer,which increases the probability of electron tunneling.

According to some embodiments, the probability of electron tunneling ina power rectifier device is significantly reduced if pits deeper thanE_(b)/F_(a) nanometers are removed. The maximum Schottky metalindentation into the semiconductor may be limited and thereby theprobability of electron tunneling, and its effect on breakdownperformance of the power rectifier device, is reduced.

The Schottky metal may for example be sputtered or evaporated titanium,tungsten, or molybdenum.

According to an embodiment, the depth of any pit of the surface of thedrift layer smaller than approximately 2 micrometers in diameter or sizemay be less than approximately 5 nanometers. Pits of such shape may notbe sufficiently narrow and deep to create an electric fieldconcentration that is high enough to eliminate the barrier height. Theembodiment is therefore advantageous in that it reduces the effect ofpitting on electrical breakdown performance of the power rectifierdevice.

According to an embodiment, the power rectifier device may include ajunction termination region at its an outer periphery, which isadvantageous in that it reduces the electric field crowding at thedevice edge and thereby reduces the risk of early electrical breakdown.The termination region may for example comprise a continuous beltprovided around the periphery of the device.

According to an embodiment, the drift layer of the power rectifierdevice includes an array of p-type regions (depletion stoppers, or fieldstoppers), which advantageously may shield the Schottky-barrier metalfrom exposure to high electric field. The p-type regions mayadvantageously be arranged in an array. Examples of p-type dopantsinclude for example aluminum and boron.

According to an embodiment, the drift layer of the power rectifier maycomprise a near-surface portion provided with a doping being 1.5 to 8times higher than the doping of the remaining part of the drift layer.The depth of the near-surface portion of the drift layer may beapproximately equivalent to the depth of the p-type depletion stoppers.

According to an embodiment, the power rectifier device mayadvantageously have an outer periphery provided with a depletion stopperregion. The depletion stopper region, such as a p-doped region, may bearranged to prevent a depletion region of the power rectifier devicefrom reaching an edge of the device during voltage blocking.

According to one embodiment, the power rectifier device may include anarray of surge pn diodes distributed within a region as defined by thejunction termination region, and wherein any of the surge pn diodes isprovided with an Ohmic contact and has a minimum lateral extension oftwo times the thickness of said drift layer.

According to one embodiment the drift layer may advantageously be ofn-type conductivity.

According to an embodiment, the step of transferring the morphologyobtained in the sacrificial layer to the surface of the drift layer mayinclude removing the sacrificial layer using an etching process. Etchingis advantageous over other material removing processes, such as forexample grinding or polishing, in that it does not expose the wafer (orat least expose it less) for mechanical working and may allow for athorough control of material removal.

Etching processes are also advantageous in that they allow for aselective planarization of the wafers. The wafer may for example beprovided with a relatively thick oxide mask that protects certainregions, such as ion implanted regions, and leaves the intended Schottkyregions exposed. In this way the implanted regions may be protectedduring the planarization such that a local planarization is obtainedwithout affecting the depth of the implanted regions. This isparticularly advantageous for SiC devices, which might have a relativelyshallow implantation depth and therefore might be sensitive forexcessive material removal.

According to an embodiment, the etching process may be a plasma etch,such as an inductively-coupled plasma (ICP) etch.

According to a further embodiment, the etching process may have aselectivity between the sacrificial layer and the SiC in the range of0.9 to 1.1. The selectivity represents the ratio of etch rates betweenthe two materials.

Using an etch process which etches SiC and the sacrificial layer atalmost the same etch rate, such as e.g. in the range of 0.9 to 1.1, isadvantageous in that it enables a transfer of the surface morphology ofthe sacrificial layer into the surface of the drift layer. During thethinning of the sacrificial layer by etching, protruding surface regionsof the drift layer will gradually be exposed to the etching process andetched at essentially the same rate as the sacrificial material.Prominent surface irregularities may thereby be replaced with thecorresponding surface morphology of the sacrificial layer, and bycontinuing the etching further also hollow irregularities, such as pitsand holes, are replaced. Providing that the sacrificial layer has asmoother morphology than the initial surface of the epitaxially growndrift layer, such a process improves the morphology of the drift layer.

According to an embodiment, the sacrificial layer may be a silicondioxide layer, which is suitable for integration in the manufacturingprocess of semiconductor devices. The oxide may for example be appliedby depositing a spin-on glass, which is advantageous in that it may beapplied in various coating thicknesses by using a process similar towhat is used for applying photoresist.

Further, the liquid nature of spin-on glass enables it to completelyfill cavities and pits having a relatively small radius of curvature. Asmaller radius of curvature increases the risk of breakthrough due toelectric field concentration. A pit having a small radius of curvaturemay on the other hand be easier to completely fill up with spin-onglass, which is advantageous in that it enhances the possibility ofremoving the pit during planarization (via etching). Using spin-on glassmight also provide a relatively smooth surface due to the forces ofsurface tension of the spin-on glass.

Other deposition techniques include for example chemical vapordeposition (CVD).

Using an oxide is also advantageous in that it may be etched bydifferent kinds of etch processes that may be used at other stages ofthe processing of semiconductor devices and which processes have a lowselectivity between oxide and silicon carbide.

Yet a further advantage with using a dielectric sacrificial layer, suchas an oxide, is that remnants after a completed etch process may berelatively easily detected. Complete removal of the sacrificial layermay for example be verified by inspection via scanning electronmicroscopy (SEM).

Examples of etch processes having a low selectivity between sacrificialoxide layer and silicon carbide include inductively-coupled plasma (ICP)etch in sulfur hexafluoride (SF₆) and argon (Ar) gas mixture, electroncyclotron resonance (ECR) plasma etch, parallel-plate reactive ion etch(RIE), and ion milling.

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise chemical mechanical polishing(CMP) of the surface of the sacrificial layer prior to transferring themorphology of the surface of the sacrificial layer to the surface of thedrift layer. The present embodiment is advantageous in that asacrificial layer having a smoother surface with fewer irregularitiessuch as for example pits and hillocks may be obtained prior to thetransfer of morphology.

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise a step of annealing the surface ofthe drift layer prior to forming the Schottky electrode on the driftlayer. The present embodiment is advantageous in that the annealing(i.e. heating) of the wafer may remove ion damage stemming from theplasma etch of the sacrificial layer and thereby provide an improveddrift layer surface having a reduced number of morphology defects thatotherwise may impair the performance of the device.

The heat treatment may for example be a rapid thermal processing (RTP).

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise the step of polishing the surfaceof the drift layer after the step of transferring the morphology of thesacrificial layer to the surface of the drift layer, and before the stepof forming the Schottky electrode. The present embodiment isadvantageous in that the polishing may further decrease the microscopicroughness of the surface of the epitaxial layer so as to form an orderedstructure of monolayer steps which might improve the depositedSchottky-barrier and reduce the amount of leakage currents.

The polishing may for example be performed by CMP.

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise (the subsequent steps of)oxidizing and hydrofluoric acid (HF) etching the surface of the driftlayer prior to forming the Schottky electrode on the drift layer. Theoxidizing may for example be performed during RTP anneal in anenvironment comprising oxygen which may react with some of the surfacematerial (Si atoms) to form silicon dioxide. The oxide may then beremoved by HF-etching, for example immediately prior to deposition ofthe Schottky metal, which is advantageous in that it provides aSchottky-barrier junction with an improved smoothness.

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise the step of implanting dopantatoms in the drift layer subsequent to the step of forming the driftlayer. The implanted regions may form for example a JTE zone, a mesh ofdepletion stoppers, and an array of pn diodes.

Performing the implantation prior to the transferring of the morphologyof the sacrificial layer to the surface of the drift layer isadvantageous in that surface damages induced by the implantation and/orirregularities induced during anneal of the drift layer might also bereduced.

According to an embodiment, the method of manufacturing a powerrectifier device may further comprise the step of implanting dopantatoms in the drift layer preceding (or before) the step of forming aSchottky electrode on the drift layer.

It will be appreciated that any of the features in the embodimentsdescribed above for the power rectifier device according to the firstaspect may be combined with the method according to the second aspect.

Further features of, and advantages with, are described in the detaileddisclosure below, the drawings and the appended claims. Those skilled inthe art will realize that different features can be combined to createembodiments other than those described in the following.

With reference to FIG. 1, there is shown a schematic view of a powerrectifier device according to an embodiment.

The power rectifier device 100 comprises a silicon carbide drift layer110 which is epitaxially grown on for example a 4H polytype substrate150 having an off-axis orientation of for example 2 to 8 degrees. ASchottky electrode 120 comprising for example titanium is disposed onthe drift layer 110. An Ohmic contact 160 is attached to the backside ofthe low-resistivity substrate 150. The drift layer 110 has a planarizedsurface (i.e. a surface being planar or flat) such that the depth of anypit 140 of the surface of the drift layer 110 is less than approximatelyD_(max)=E_(b)/F_(a), where E_(b) is the metal-semiconductor barrierheight and F_(a) is the avalanche breakdown field.

A pit 140 of the surface of the drift layer 110 may cause the formationof a metallic needle propagating into the semiconductor upon metaldeposition. A small-radius metal tip might lead to a high localconcentration of the electric field which generally increases with adecreasing radius of curvature of the metal indentation. A highprobability of electron tunneling from the Schottky metal 120 into thesemiconductor may limit the thermal barrier for current flow from themetal to the semiconductor and hence the effective barrier energy islowered. The energy barrier between the metal and the semiconductor mayhowever be maintained as long as the maximum decrease of the barrierheight due to the metal indentation does not exceed themetal-semiconductor barrier height E_(b). The maximum mean electricfield in the semiconductor is limited by the avalanche breakdown field,F_(a). Any metal indentation being not deeper than D_(max)=E_(b)/F_(a)may therefore not decrease the barrier height to zero.

Values of the metal-semiconductor barrier height E_(b) and the avalanchebreakdown field F_(a) might for example be 1 eV and 2 MV/cm,respectively. Hence, according to an embodiment, any pit no deeper thanapproximately 5 nm might ensure a negligible (or at least significantlyreduced) effect of pitting on the breakdown performance of the powerrectifier device 100.

Shallow indentations, having a depth of less than 5 nm and a lateraldimension (or width) above approximately 2 μm might, according to thepresent embodiment, retain on the surface due to their relatively largeradius of curvature. With regard to FIG. 1, it can be noted that thedepth of the remaining pit 140 on the surface of the drift layer is notto scale by orders of magnitude. The drift layer may be provided athickness of approximately 0.7 to 1.1 μm per 100V of desired voltage.Advantageously, the doping of the drift layer 110 may be sufficientlylow to provide a maximum electric field at the rated blocking voltagebelow the critical field of avalanche breakdown in 4H SiC.

Advantageously, the device periphery of a high power rectifier may beprotected from electric field crowding effects. As shown in FIG. 2, theouter periphery of the power rectifier device 200 may be provided withan ion-implanted p-type depletion stopper region 212 and a junctiontermination (JT) region 211 that may suppress the spike of electricfield at the outer periphery of the power rectifier device 200. Bothregions 211, 212 may be formed as a continuous belt enclosing the powerrectifier device 200.

A p-type Ohmic contact 213 may be provided to the depletion stopperregion 212, which advantageously may enable for the potentials of theSchottky metal 120 and of the inner periphery of the JT region 211 to beapproximately equal. The outer periphery 221 of continuous Schottkymetal (only the outline 221 is shown in FIG. 2) may fully overlap thesurface of the drift layer 210, and may further be located within theOhmic contact region 213.

The JT region 211 may be formed, for example by implanting acceptorswith a dose of approximately 10¹³ cm⁻², so as to form a junctiontermination extension (JTE). It will be appreciated that other junctiontermination techniques than the use of a JTE may be applied. As anexample, an array of floating guard rings may be used as a junctiontermination.

With reference to FIG. 3, the power rectifier device may be providedwith a closely spaced array of ion-implanted p-type depletion stoppers314 underneath the Schottky metal 120. The closely-spaced array mayprovide electrostatic shielding to the Schottky-barrier. The electricfield at the metal-semiconductor interface of the device according tothis embodiment may advantageously be lower than that in a non-shieldedpower rectifier device. Reverse currents in SiC Schottky diodes aregoverned by tunneling, and therefore decreasing the electric field atthe Schottky interface might be advantageous. A close spacing betweenadjacent depletion stoppers 314 may enable electrostatic shielding.

Advantageously, the maximum spacing between said closely spaced adjacentdepletion stoppers may not exceed approximately 6 times the p-dopantpenetration depth, which might provide substantially high shielding.This relation may for example correspond to a range of the spacingbetween adjacent p-type depletion stoppers 314 of approximately 1 to 5micrometers, depending on the penetration depth in SiC.

The top portion of the drift layer 110 in the shielded power rectifierdevice design 300 may have a function similar to that of channels invertical field-effect transistors. The total area of the channels may besubstantially smaller than the total Schottky metal area, as a portionof the total area may be consumed by the depletion stoppers 314. Anadditional portion of the useful power rectifier device cross-sectionarea might be consumed by the regions adjacent to the depletion stoppers314 due to said adjacent regions being depleted by the built-inpotential of the pn junctions. Advantageously, the near-surface portionof the of the drift layer 110 may have a doping level increased by afactor of 1.5 to 8 as compared to the doping level of the main body ofthe drift layer 110. The thickness of said near-surface channel portionmay be approximately equal (close) to the implant depth of the p-typedepletion stoppers 314.

Advantageously, the width of the ion-implanted p-type depletion stoppers314 may not exceed the spacing between them since the device areaconsumed by the depletion stoppers 314 may not be used for verticaltransport of electrons from the anode to the cathode.

The power rectifier device 300 may further be provided with an Ohmiccontact 160 at the backside of the substrate 150. The epitaxial layerstack may further comprise a buffer layer 170 which may suppress theeffect of substrate crystal imperfection on crystal quality of the driftlayer 110.

According to one embodiment, the power rectifier device may be furtherprovided with a number of relatively large surge-current pn diodes(surge diodes) 315 distributed over the area of the drift layer 110. Thesurge diodes 315 may utilize the same type of p-implant and Ohmiccontact as the outer p-type region 312. All the surge diodes 315 may befully covered by the Schottky-barrier metal.

The device 300 may be provided with further safety features for the caseof current surge conditions. A pn diode portion along the rim of devices200 and 300 may provide such safety feature, since a pn junction mayretain a relatively low forward drop under high current densities due tominority carrier injection. However, the total area of the rim might berelatively small, which may enable the device 200 of being capable ofmaintaining a relatively low current surge. The array of surge diodes315 may distribute the surge current over a much larger area, whichtherefore might provide a device 300 with higher stability to currentsurge. Advantageously, the smallest lateral dimension of a surge diode315 may exceed two (2) times the thickness of the drift layer 110. Surgediodes of smaller area may be shorted by adjacent Schottky-barrierregions, which in silicon carbide may have smaller turn-on voltage thanthe pn diode.

The fraction of Schottky-barrier diode area utilized by the surge diodes315 may be chosen according to application-specific requirements. Toodense a surge diode array might consume a high percentage of the driftlayer area, whereas too loose an array might have low value ofacceptable surge current.

The surge diode array is not limited to an array of circular diodes.Different configurations of surge diodes may be applied, such as forexample a linear array of pn diode stripes having a stripe widthexceeding two (2) times the thickness of the drift layer.

The power rectifier device may comprise closely spaced depletionstoppers 314 or an array of surge diodes 315 distributed over theSchottky diode area, or a combination of both as shown in FIG. 3. Thedevice may also be provided with a JTE region along the entireperiphery.

FIG. 4 shows a cross section of a power rectifier device having abackside Ohmic contact 460, provided on the substrate 450, and a bufferlayer 470 provided between the substrate 450 and the drift layer 410. Adedicated array of surge diodes 415, having a minimum dimension of asurge diode 415 exceeding approximately two (2) times the drift layer410 thickness, may be provided in the near-surface portion 416 of thedrift layer 410 in order to provide improved protection against surgecurrent conditions. Each surge diode 415 may be provided with an Ohmiccontact.

FIGS. 5 a to 5 d schematically illustrate an example embodiment of amethod of manufacturing a power rectifier device.

In FIG. 5 a, a drift layer 510 including SiC is provided. The driftlayer may be epitaxially grown on a SiC substrate 150. The top surfaceof the drift layer 510, onto which the Schottky electrode 120 will beprovided, comprises irregularities such as pits 540 and steps 542 formedfor example during epitaxial growth of the drift layer 510 and duringsubsequent processing of the substrate 150. The irregularities mightincrease the risk of leakage currents caused by electron tunneling dueto local concentration of the electric field.

As illustrated in FIG. 5 b, a sacrificial layer 522, such as for exampleSiO₂, may be provided by deposition of a spin-on glass on the surface ofthe drift layer 510. Spin-on glass is a type of glass that can beapplied as a liquid and cured to form an oxide layer on the surface. Dueto its liquid characteristics, the spin-on glass may fill the cavitiesof the drift layer 510 and provide a smoothened surface. Spin-on glasslayers can be obtained with a coating thickness of about 50 nm. However,both thinner and thicker coatings may be used for forming thesacrificial layer 522.

The spin-on glass may be applied using a technique similar toconventional application of photoresist, i.e. spinning and baking,followed by a subsequent curing step.

The formation of the sacrificial layer 522 may be followed by alow-selectivity plasma etch, such as for example an inductively-coupledplasma (ICP) etch in SF₆ and Ar gas mixture. Accordingly, SiC and SiO₂may be etched at substantially the same etch rate, which enables atransfer of the morphology obtained on the sacrificial layer 522 to thesurface of the drift layer 510.

As shown in FIG. 5 c, any protruding parts of the surface of the driftlayer might eventually be exposed to, and etched by, the plasma duringthe progress of the etching process.

FIG. 5 d shows a planarized surface wherein the etching process hascontinued until the sacrificial layer 522 has been removed from thedeepest indentation. All irregularities but the lower part of the pit540 is removed. As a result, the surface is smoother than before theplanarization was initiated and in particular smoother than as-grown.The present method of manufacturing is advantageous on that it may onlyretain pits 540 no deeper than approximately 5 nm (indicated d₁ in FIG.5 d), which have a reduced effect on the resulting breakdown performanceof the power rectifier device 100. The surface morphology of thesacrificial layer 522 has been transferred to the surface of the driftlayer 510.

The planarization as described above may also be repeated in order tofurther enhance the surface smoothness, which is particularlyadvantageous if an etching process having a higher selectivity betweenthe sacrificial layer 522 and the SiC, such as for example 0.7, is used.

FIGS. 6 a and 6 b show a surface of the drift layer 610 having a 40 nmdeep pit 640. A single planarization cycle using a plasma etch having anoxide-SiC selectivity of 0.9 will decrease the depth d₀ of the pit 640to approximately 4 nm (FIG. 6 b) which may be sufficient to eliminatethe undesirable electric field concentration effects. By repeating theprocedure, i.e. adding and etching a second sacrificial layer 622 on topof the drift layer 610, the pit depth d₁ might decrease further.Optionally, the number of planarization cycles may be further increasedif required. For example, this might be advantageous if the selectivityof the planarization etch (substantially) deviates from 1.

After the planarization etch in which the morphology of the sacrificiallayer 622 is transferred to the surface of the drift layer 610, ascanning electron microscope (SEM) may be used to verify that all oxide622 has been removed.

Actual depth of the remained pits 640 may be monitored utilizingcharacterization techniques such as for example Atomic Force Microscopy(AFM) or Tunnel Microscopy.

FIG. 7 is a block diagram schematically showing the method ofmanufacturing a power rectifier device according to an embodiment.

A drift layer is formed 7001 on a wafer comprising a substrate asdescribed above. The formation 7001 may be followed by an implantationstep 7010, wherein for example aluminum may be ion implanted to formp-type regions in the drift layer.

A sacrificial layer is then formed 7002 on the drift layer and may beCMP polished during a polishing step 7020 in order to further improvethe morphology of the surface such that a smoother surface with reducedirregularities is formed.

A step of etching 7003, or transferring of the morphology of thesacrificial layer into the drift layer, may be followed by an inspectionstep 7030 using SEM. This inspection step 7030 may be added to verifyremoval of the sacrificial layer.

For further reducing any surface irregularities and damages that mighthave been induced by the etching process an anneal step 7004 may followthe etch process. The wafer may be heated to a temperature between 900°C. and 1300° C. in an environment containing oxygen such that thesurface becomes oxidized. If the surface is a silicon crystal face ofthe SiC, the oxide may be for example 1-2 nm, while it may be a few tensof nanometers or thicker for a carbon face of SiC. The oxide may then beremoved by an HF etch 7005.

The metal deposition 7006 may be preceded by an ion implantation 7040 ofaluminum, wherein a mesh of p-type depletion stoppers and/or an array ofpn diodes are formed in the drift layer. The surface may also bepolished 7050 to reduce any remaining defects, wherein for example 10-20nm of the surface is removed.

In one example, the manufacturing of a pn diode comprises the steps ofgrowing an n-type SiC layer on a p-substrate having an etched trench,deposition of sacrificial oxide, CMP of the oxide, and planarizationetch. A smoother pattern obtained by dishing of the oxide in course ofCMP in the centre of the trench may be transferred into the SiC.

In one example, a CVD oxide having a thickness of 100-200 nm may bedeposited and patterned so as to mask the implanted p-type layer. Spinon-glass having a thickness of 60 nm may then be deposited and baked at250° C. and followed by a planarization etch to remove the spin-on glassin the central part of the device. Ion damage might then be annealed,and backside Ohmic contact provided by cleaning the wafer backside fromoxide, depositing nickel and sintering it at 960° C.

The remaining oxide may then be stripped in HF and followed by anneal ofimplantation. Optionally, the surface may be further improved by CMPaccording the embodiment, described above, wherein the method ofmanufacturing the power rectifier device further comprises polishing thesurface of the drift layer after the step 7003 of transferring themorphology of the sacrificial layer to the surface of the drift layer.

Ti Schottky metal may then be deposited, followed by application of Albonding pad metal to the front side (device side), and application ofgold solder metal to the back side.

A planarization of implanted SiC surface according to embodiments isadvantageous, since it enables removal of only a small thickness of SiC.As a result, a thorough control of material removal may be achieved inorder to not affect the depth of implanted p-well too much.

Planarization may be performed in two stages, wherein the first stageremoves growth pits 10 on the as-grown epitaxial wafer and the secondplanarization stage may be applied after anneal of the acceptor implantso as to remove the surface imperfections that might have appeared as aresult of the activation anneal. The shielded design of the deviceaccording to this embodiment may favor the use of a metal having a lowerbarrier to SiC, such as tungsten (W) or molybdenum (Mo). Such metalsmight result in a barrier height of approximately 800 mV compared to the1200 mV barrier height provided by Ti annealed at 400-450° C. The lowerbarrier height of W or Mo may result in a lower forward voltage drop. Alower current leakage may be achieved via the electrostatic shieldingcombined with the locally planarized surface.

In one example, an array of pn diodes directed in parallel to the powerrectifier device 1 may cover approximately 10% to 30% of theSchottky-barrier area.

A high-dose implant 7010 with a dose above 1×10¹⁴ cm⁻² may be performedto define the pn diodes and the pn diode rim along a periphery of theSchottky-barrier area. Another implant may be performed to define theJTE region 311 at the rectifier periphery. The width of the JTE 311 maybe approximately 20-60 μm, or at least twice the width of the depletionregion at the maximum blocking voltage. A metal contact may overlap theJTE 311 by at least a few micrometers. The JTE 311 may comprise a p-typelayer with a dopant dose of electrically active acceptors ofapproximately 1.1×10¹³ cm⁻². The dopant may for example be aluminumwhich is ion implanted using an implant energy of 300 keV and animplanted dose of 1.65×10¹³ cm⁻². The implant anneal may be performed at1650° C. for 30 minutes under a carbon capping layer that may be formedby thermal treatment at for example 800° C. of a hard-baked photoresist.After the implantation, the carbon cap may be removed in oxygen plasma.After strip of the carbon layer, a local planarization can be performedas described above in connection to for example FIGS. 5 and 7. Anoptional CMP planarization step 7040 may be added to further improve thesurface morphology.

The pn diode regions may be masked with approximately 200 nm thick oxideso as to avoid undesired removal of the p-type material. The mask forthis thicker oxide could be offset by approximately 2-3 μm towards thecentral part of each p-type region so as to avoid undesired masking ofthe n-type region.

The backside Ohmic contacts 160 may be formed by sintering Ni aspreviously described. Wells may be opened in the oxide on the top side(device side) of the wafer in the areas that will be provided with Ohmiccontacts. An Al/Ti metal stack may then be deposited and patterned so asto define the Ohmic contacts. The Al/Ti stack may be sintered atapproximately 950° C. to form the Ohmic contacts. The compound providingOhmic behavior of the Al/Ti contact is known to be due to formation ofintermetallic compound Ti₃SiC₂, which may be lattice matched to SiC. Thesacrificial oxide 522 may at this stage be fully removed from the topsurface in buffered HF, after which the substrates are transferred intoa deposition chamber wherein the Ti Schottky metal 120 may be deposited.Device manufacture may then be finalized by depositing and patterning Alpad metal on the top. A silver solder metal may be applied to the waferbackside. The device can also be protected by polyimide.

In another example, a semiconductor template for Schottky-barrier powerrectifier manufacture may be provided with a locally planarized surfaceshortly after epitaxial growth. Local planarization may remove pitshaving a depth approximately greater than 5 nm, as described in thepreceding embodiments. Such a procedure is advantageous in that astarting material having a reduced number of morphology imperfectionsmay be obtained for the manufacturing of Schottky-barrier powerrectifiers. Epitaxial wafers according to this embodiment areadvantageous in that they may simplify the manufacturing ofSchottky-barrier power rectifiers. Silicon carbide wafers may often bedefective around the wafer edges, and therefore certain edge exclusionmay apply. Substrate regions, typically a few millimeters from the waferedge, may in many cases not meet the requirements for crystal or surfacequality. The present embodiment is advantageous since silicon carbidewafers may contain a certain amount of rough defects, which may causeinevitable failure of any power device incorporating said rough defects.It is not a requirement to fully planarize (or flatten) said roughdefects for obtaining the benefits of the present embodiment.

Generally, embodiments may result in semiconductor wafers provided witha low-doped (i.e. 3×10¹⁴ to 6×10¹⁶ cm⁻³) epitaxial layer having athickness of approximately between 4 and 100 micrometers and a donordoping level corresponding to a theoretical breakdown voltage betweenapproximately 300V and 15 kV. As these layers may suffer much less frompits and other defects, the resulting breakdown voltage corresponds tothe breakdown voltage that can be calculated using for thestructure-specific doping profiles with use of already known impactionization rates in 4H SiC.

In one general aspect, a power rectifier device for power conversionapplications can include a drift layer including silicon carbide, and aSchottky electrode disposed on the drift layer. The Schottky electrodeand a surface of the drift layer providing a Schottky contact, where thedrift layer has a planarized surface such that the depth of any pit ofthe surface of the drift layer is less than approximatelyD_(max)=E_(b)/F_(a), where E_(b) is the metal-semiconductor barrierheight and F_(a) is the avalanche breakdown field.

In some implementations, the depth of any pit of the surface of thedrift layer smaller than approximately 2 micrometers in diameter or sizeis less than approximately 5 nanometers.

In some implementations, the power rectifier device can includeincluding a junction termination region at its outer periphery.

In some implementations, the power rectifier device can be provided withan array of p-type depletion stoppers.

In some implementations, a near-surface portion of the drift layer isprovided with a doping being 1.5 to 8 times higher than the doping ofthe remaining part of the drift layer, and the depth of the near-surfacedrift layer portion is approximately equivalent to the junction depth ofthe p-type depletion stoppers.

In some implementations, the power rectifier can be provided with anarray of surge pn diodes distributed within a region as defined by thejunction termination region, where any of the surge pn diodes isprovided with an Ohmic contact and has a minimum lateral extension oftwo times the thickness of the drift layer.

In another general aspect a method of manufacturing a power rectifierdevice can include forming a drift layer including silicon carbide,forming a sacrificial layer on a surface of the drift layer,transferring the morphology obtained in the sacrificial layer to thesurface of the drift layer, and forming a Schottky electrode on thedrift layer. The Schottky electrode and the surface of the drift layercan provide a Schottky contact.

In some implementations, the transferring of the morphology obtained inthe sacrificial layer to the surface of the drift layer includesremoving the sacrificial layer using an etching process.

In some implementations, the etching process is a plasma etch.

In some implementations, the etching process has a selectivity betweenthe sacrificial layer and the silicon carbide in the range of 0.9 to1.1.

In some implementations, the sacrificial layer is an oxide layer.

In some implementations, the method can include polishing of the surfaceof the sacrificial layer prior to transferring the morphology of thesacrificial layer to the surface of the drift layer.

In some implementations, the method can include annealing the surface ofthe drift layer prior to forming the Schottky electrode on the driftlayer.

In some implementations, the method can include polishing the surface ofthe drift layer after transferring the morphology of the sacrificiallayer to the surface of the drift layer, and before forming the Schottkyelectrode.

In some implementations, the method can include oxidizing and HF-etchingthe surface of the drift layer prior to forming the Schottky electrodeon the drift layer.

In some implementations, the method can include implanting (7010) dopantatoms in the drift layer subsequent to the forming of the drift layer.

In some implementations, the method can include implanting dopant atomsin the drift layer preceding the forming of a Schottky electrode on thedrift layer.

In yet another general aspect, a semiconductor product, can include ann-type silicon carbide substrate having a lightly doped epitaxial n-typedrift layer disposed on top of the substrate, where the surface of theepitaxial layer is locally planarized such that any pit is less than 5nanometers, with a lateral extension of the pit being less than 2micrometers.

In some implementations, the n-type silicon carbide substrate is a4H-silicon carbide substrate.

While specific embodiments have been described, the skilled person willunderstand that various modifications and alterations are conceivablewithin the scope as defined in the appended claims.

What is claimed is:
 1. A power rectifier device, comprising: a driftlayer including silicon carbide; and a Schottky electrode disposed onthe drift layer, the Schottky electrode and a surface of the drift layerproviding a Schottky contact, the drift layer having a planarizedsurface such that a depth of each of a plurality of pits of the surfaceof the drift layer is less than approximately D_(max)E_(b)/F_(a), whereE_(b) is a metal-semiconductor barrier height and F_(a) is an avalanchebreakdown field.
 2. The power rectifier device of claim 1, wherein anypit of the surface of the drift layer having a diameter or lateralextension of less than approximately 2 micrometers has a depth which isless than approximately 5 nanometers.
 3. The power rectifier device ofclaim 1, further comprising a junction termination region at an outerperiphery of the power rectifier device.
 4. A semiconductor product,comprising: an n-type silicon carbide substrate; and a lightly dopedepitaxial n-type drift layer disposed on top of the n-type siliconcarbide substrate, the lightly droped epitaxial layer having a surfacelocally planarized such that each of a plurality of pit having adiameter or lateral extension of less than 2 micrometers has a depth ofless than 5 nanometers.
 5. The semiconductor product of claim 4, whereinthe n-type silicon carbide substrate is a 4H-silicon carbide substrate.6. The power rectifier device of claim 1, wherein the Schottky electrodeincludes a metal.
 7. The power rectifier device of claim 1, wherein theSchottky electrode includes one of titanium, tungsten or molybdenum. 8.The power rectifier device of claim 1, wherein the drift layer is alayer grown by epitaxy on a 4H polytype substrate having an off-axisorientation of 2 to 8 degrees.
 9. The power rectifier device of claim 3,wherein the junction termination region is formed as a continuous beltenclosing the power rectifier device.
 10. The power rectifier device ofclaim 1, further comprising an ion-implanted p-type depletion stopperregion at an outer periphery of the power rectifier device.
 11. Thepower rectifier device of claim 10, wherein the p-type depletion stopperregion is formed as a continuous belt enclosing the power rectifierdevice.
 12. The power rectifier device of claim 10, wherein a p-typeOhmic region is provided to contact the p-type depletion stopper region.13. The power rectifier device of claim 12, wherein the Schottkyelectrode is located within the p-type Ohmic region.
 14. The powerrectifier device of claim 1, wherein an outer periphery of the Schottkyelectrode fully overlaps the drift layer.
 15. The semiconductor productof claim 4, further comprising a Schottky electrode disposed on then-type drift layer.
 16. The semiconductor product of claim 15, whereinthe Schottky electrode and the drift layer together form part of adiode.